Nyquil is an over-the-counter medication used to treat symptoms of the common cold, flu, and other upper respiratory infections. It is a combination of several active ingredients, including:

Acetaminophen: This is a pain reliever and fever reducer.

Dextromethorphan: This is a cough suppressant that helps to control coughing.

Doxylamine succinate: This is an antihistamine that causes drowsiness and is used as a sedative to help with insomnia and to provide relief from symptoms such as runny nose, sneezing, and itchiness.

Nyquil is typically used to relieve symptoms such as fever, headache, muscle aches, sore throat, coughing, congestion, and sneezing. It is intended for use as a night-time sleep aid and is not recommended for use during the day because of the drowsiness-inducing effects of the doxylamine succinate. Nyquil stays in your system depending on the dosage that you have taken and other factors.

It's important to read the label and follow the recommended dosage, as taking more than the recommended amount of Nyquil can cause liver damage. Also, it's not recommended for children below the age of 12, and should not be mixed with alcohol or other medications.